Make valloc() get page size through sysconf(), add tests

See merge request !216

Implement LCG pseudorandom number functions

See merge request !213

Add memrchr()

See merge request !214

Spring cleanup

See merge request !217

The race condition didn't trigger when I tested it, so I assumed I had
solved it...

Some recipes require the headers, but they don't explicitly say
so. Parallel make (-j`nproc`) might start compiling the libs when the
headers aren't done yet.

Add completely untested redox epoll implementation

See merge request redox-os/relibc!211